A variety of methods are used for recovering crude oil from subterranean formations. Initially, oil is produced from a formation by pressure depletion. In this method, the differential pressure between the formation and a production well or wells forces the oil contained within the formation toward a production well where it can be recovered. Typically, up to about 35 percent of the oil which is initially contained in a formation can be recovered using pressure depletion. This leaves a large quantity of oil within the formation. Additionally, some formations contain oil which is too viscous to be efficiently recovered from the formation using pressure depletion methods. Because of the need to recover a larger percentage of the oil from a formation, methods have been developed recover oil which could not be recovered using only pressure depletion techniques. These methods are typically referred to as “enhanced oil recovery techniques”.
Previous oil production methods leave as much as 50% of the original oil in place. Recent efforts to recover that oil remaining in the reservoirs have had considerable success. Among the more promising of the methods being used today is an enhanced oil recovery process referred to as a surfactant flood.
An aqueous fluid containing surfactant is injected into an oil rich formation to displace oil from the formation and the displaced oil is then recovered.
In oil-wet formations, such as carbonate rock formations, the hydrophobic surface of the formation limits imbibition of aqueous fluid into the formation and thus limits the amount of oil that can be displaced by the flooding method.
Most of the carbonate reservoirs are naturally fractured and recovery from these reservoirs is typically much lower than that from unfractured reservoirs. According to a recent review of 100 fractured reservoirs, Enhanced Oil Recovery (EOR) technique is requested to achieve maximum production of such reservoirs with high matrix porosity and low matrix permeability. The wettability of originally water-wet carbonate reservoirs is altered by the adsorption of polar compounds and/or deposition of organic matter that was originally in the crude oil and many carbonate reservoirs are thus mixed-wet or oil-wet. Although it is difficult to identify precisely which compounds are critical in altering the wettability of the reservoir it has been suggested that asphaltene, naphtenic acid and a number of carboxylic acids including caprylic, palmitic, stearic and oleic were the main cause. These natural surfactants in crude oil are sufficiently soluble in water to pass through a thin layer of water and adsorb onto the rock surface. The main difference between silica and calcite is that the point of zero charge for the calcite is approximately 8.2 and thus surface of calcite is positively charged at neutral pH. If rocks are oil-wet or mixed-wet, spontaneous imbibition does not occur. The use of surfactant to alter the rock surface to increase water-wetness, decrease interfacial tension and promote imbibition of water into the matrix reservoir has been proposed to improve oil recovery. However, the use of surfactants is not always satisfactory in carbonate formations.
The main mechanisms to improve recovery in oil-wet formations are: wettability alteration and wettability alteration and interfacial tension reduction
With respect to the wettability alteration two different strategies can be considered: Such strategies relate to chemical compounds that will adsorb directly on the organic carboxylates deposited on the carbonate surface of the reservoir and will increase the hydrophilicity of the surface and chemical compounds that will desorb the adsorbed organic carboxylates and render the surface more hydrophilic.
To reach the solid-liquid interface, diffusion through the water-oil interface and then through the oil phase represents the only transport mechanism. This suggests that small and non associated molecules are probably better candidates compare to larger macromolecules.
In general anionic surfactants are not able to desorb organic carboxylates adsorbed to a chalk surface.
Cationic surfactants of the type R—N+(CH3)3 are able to desorb organic carboxylates from the chalk surface in an irreversible way and 70% of the oil in place can, in certain circumstances, be recovered within 30 days by spontaneous imbibition of the aqueous surfactant solution at 70° C. In that case, ion-pairs are formed and dispersed in the oil phase. Use of cationic surfactants may change wettability. However, cationic surfactants may be incompatible with anionic surfactants typically used to decrease interfacial tension to also assist in enhancing oil recovery from subterranean formations.